1. Field of the Invention
The present invention relates to a silica glass crucible and a method of fabricating thereof. More particularly, the present invention relates to a silica glass crucible capable of enhancing the yield in conversion to single crystal during the withdrawal of single silicon crystal and a process for the production thereof.
2. Description of the Related Art
Single silicon crystal to be incorporated in the substrate of semiconductor device is usually prepared by Czochralski process (Cz process). In accordance with CZ process, a silica glass crucible is loaded with polycrystalline silicon as a starting material. The silicon is then circumferentially heated so that it is fused. A hanging seed crystal is allowed to come in contact with the silicon melt, and then withdrawn therefrom.
The conventional silica glass crucible is produced by vacuum fusion or the like. When the temperature of the silica glass crucible is raised during the withdrawal of single crystal, bubbles are produced in the transparent layer, causing the deformation of the silica glass crucible. Further, the silicon melt erodes the surface of the silica glass crucible. The erosion causes bubbles in the transparent layer (inner layer) of the silica glass crucible to be exposed at the interface with the silicon melt, rendering the conversion to single crystal unstable. As a result, the yield in the conversion to single crystal is lowered. Bubbles produced in the transparent layer are mainly composed of oxygen. Portions containing much impurities or structural water nucleate for bubbles. Surrounding oxygen diffuses into the bubbles thus formed to cause the bubbles to grow.
In recent years, various preparation processes have been studied for preparing a silica glass crucible with its transparent layer free of bubbles. However, none of these proposals allow total elimination of bubbles. Even if these proposed processes allow remarkable reduction of the production of bubbles in the transparent layer as compared with the conventional silica glass crucible, the yield in conversion to single silicon crystal which is required to have a high quality cannot necessarily be sufficiently enhanced to a satisfactory extent.
A crucible preparation process has heretofore been commonly practiced which comprises fusing a quartz powder as a starting material in a mold under reduced pressure to form a transparent layer on the inner side of a crucible. In this preparation process, however, bubbles are left in the transparent layer in the vicinity of the inner surface of the crucible. When a single crystal is withdrawn from the crucible prepared by such a preparation process, bubbles are expanded in the transparent layer. As the transparent layer on the inner side of the crucible is dissolved away, bubbles enter into the silicon melt. Thus, bubbles are taken into the single silicon crystal thus withdrawn from the crucible. The bubbles cause crystal dislocation resulting in dislocation (crystal defect), lowering the percent conversion to single crystal.
Various preparation processes have been studied for preparing a silica glass crucible with its transparent layer free of bubbles.
For example, as a process for the preparation of a silica glass crucible having less bubbles in the transparent layer there is described in JP-A-1-157426 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) a process which comprises supplying a quartz powder as a starting material into an air-permeable mold to form a crucible-shaped molded product, reducing the pressure in the system, and then fusing the material with helium gas, argon gas or a mixture thereof being supplied thereinto to prepare a crucible. In accordance with the preparation process, a crucible can be prepared with its transparent layer having somewhat less content of bubbles. However, when a single crystal is withdrawn from this silica glass crucible, bubbles are expanded as in the silica glass crucible prepared by fusing a quartz powder as a starting material at the atmospheric pressure. The bubbles are then taken into the single silicon crystal thus withdrawn, causing crystal dislocation resulting in dislocation and hence making it impossible to provide sufficient enhancement of percent conversion to single crystal.
Further, JP-A-1-157427 discloses a process which comprises supplying a quartz powder as a starting material into an air-permeable mold to form a crucible-shaped molded product, reducing the pressure in the system, and then supplying hydrogen gas, helium gas or a mixture into the system from the initiation of fusion to prepare a crucible. In accordance with this preparation process, hydrogen gas and helium gas diffuse into the transparent layer of the crucible, making it for other gases to diffuse into the transparent layer. Thus, the gases in the bubbles formed during fusion diffuse away out of the silica glass. However, when a crucible is prepared by supplying helium gas from the initiation of fusion until the termination of fusion, the same problems as in the foregoing preparation processes occur. When a crucible is prepared by supplying hydrogen gas or a mixture of hydrogen gas and helium gas from the initiation of fusion until the termination of fusion, the expansion of bubbles during the withdrawal of single crystal can be inhibited, but the OH concentration in the silica glass crucible rises to lower the viscosity of the silica glass, occasionally causing the deformation of the crucible. This disadvantage becomes remarkable particularly with large-sized silica glass crucibles intended for prolonged use.
Further, JP-A-7-330358 discloses a process for the preparation of a silica glass crucible which comprises cooling the inner surface of a silica glass crucible with a hydrogen atmosphere after the termination arc fusion.
In accordance with this preparation process, the production and growth of microbubbles can be inhibited. However, since the supply of hydrogen is effected after the termination of arc fusion, it is made difficult to reduce the amount of bubbles which are not a little left in the transparent layer of the crucible and prevent bubbles in the outer opaque layer from being expanded.
Moreover, JP-A-5-124889 discloses a process which comprises subjecting a crucible formed by arc fusion (which has been allowed to cool to room temperature) to heat treatment at a temperature of from 300xc2x0 C. to 1,200xc2x0 C. in a hydrogen atmosphere. In accordance with this preparation process, the problem of expansion of bubbles in the crucible can be solved. However, the inner surface of the crucible is eroded considerably partially during the withdrawal of single crystal, resulting in the drop of percent conversion to single crystal.
It has been desired to provide a silica glass crucible which allows the withdrawal of a single crystal at a high percent conversion to single crystal without taking bubbles into the single silicon crystal thus withdrawn and causing dislocation, can be used over an extended period of time and can be obtained in a large size and a process for the preparation thereof in a high yield.
The present invention has been worked out under the foregoing circumstances. An object of the present invention is to provide a silica glass crucible which allows the withdrawal of a single crystal at a high percent conversion to single crystal without taking bubbles into the single silicon crystal thus withdrawn and causing dislocation, can be used over an extended period of time and can be obtained in a large size and a process for the preparation thereof in a high yield.
A first implementation of the present worked out to accomplish the foregoing object is a silica glass crucible having a of 0.05 or less over a thickness of 0.5 mm or more from the inner surface of the silica glass crucible, xcex1 being obtained by dividing the fluorescent intensity integrated over a wavelength range of 4,000 cmxe2x88x921 and 4,100 cmxe2x88x921 by the fluorescent intensity integrated at a wavelength of 800 cmxe2x88x921 where an SiO peak appears as determined by subjecting a section of the thickness of the silica glass crucible to laser Raman spectroscopy involving excitation by laser beam of 514 nm, and a hydroxyl group concentration of 100 ppm or less over the entire periphery beyond a thickness of at least 1.0 mm from the inner surface of the silica glass crucible.
A second implementation of the present invention is a silica glass crucible according to the first implementation, substantially free of micro-cracking on the inner surface thereof.
A third implementation of the present invention is a process for the preparation of a silica glass crucible for withdrawing single silicon crystal which comprises supplying a quartz powder as a starting material into a rotating mold to form a crucible-shaped molded product which is then arc-fused, characterized in that the supply of helium gas or argon gas at least into the interior of said crucible-shaped molded product is followed by the initiation or continuation of arc fusion before the termination of which the supply of argon gas is suspended or the supplied amount of argon gas is reduced and the supply of hydrogen gas is initiated.
A fourth implementation of present invention is a process for the preparation of a silica glass crucible for withdrawing single silicon crystal according to the third implementation, wherein the reduction of pressure is effected on the peripheral side of the crucible-shaped molded product via the mold before or at the same time with said arc fusion or after the fusion of the inner surface of the crucible-shaped molded product by the arc fusion.
A fifth implementation of the invention is a process for the preparation of a silica glass crucible for withdrawing single silicon crystal according to the fourth implementation, wherein the reduction of pressure is lessened or suspended before the initiation of the supply of hydrogen gas.
The sixth implementation of the present invention is a process for the preparation of a silica glass crucible for withdrawing single silicon crystal according to the fourth implementation, wherein the supply of hydrogen gas is initiated at latest 5 minutes before the termination of arc fusion.
The seventh implementation of the present invention is a process for the preparation of a silica glass crucible for withdrawing single silicon crystal according to the sixth implementation, wherein the supply of hydrogen gas is initiated after the elapse of time corresponding to 40% of the total arc fusion time from the initiation of arc fusion.